Fluid properties sensors are known that make use of the interaction of a vibrating element with the fluid in which it is in contact. Among those sensors are those that make use of torsional resonators, and among those torsional resonator systems are those that are both driven and sensed by means of electromagnetic transducers.
Compared to piezoelectrically driven and sensed resonators, those with magnetic transducers are more robust and more stable over time, particularly with respect to temperature variations. In particular, sensors that are used in extreme environments, as for downhole fluid measurement in oil, gas, and geothermal drilling and well logging, may be exposed to pressures in excess of 2000 bar and temperatures of 200° C.
Patent application WO/2012/012508 describes coupled torsional resonators that incorporate magnets in their resonant structures that are driven and sensed by coils external to the resonator. The resonator is immersed in the fluid, while the coils may be placed outside the conduit containing the fluid, possibly in a chamber that is nominally at atmospheric pressure, so that the coils and their electrical connections are protected from the fluid. This obviates the need for electrical pressure feed-throughs and other sealing devices that are susceptible to degradation and failure.
Patent application WO/2012/012508 and related filings disclose means for exciting and sensing vibrations of the torsional resonator by means of either permanent magnets, or soft magnetic materials embedded in the oscillating parts of the sensor. The disadvantage of permanent magnets is that they may attract magnetic particles from the fluids in which they are immersed, which then adhere to the surface of the sensor and distort said sensor's readings.
Another method for measuring viscosity and possibly other properties of fluids is disclosed in US patent application 2013/0167620. That method includes a torsional resonator whose vibrations are excited by and sensed by a coil acting upon and acted upon by the field of a permanent magnet attached to the resonator, with the coils being a short distance away from the resonator.
In these and many other embodiments of electromagnetically sensed resonators, it is advantageous to make the sensing system as sensitive as possible in order to be able to measure the smallest expected amplitudes of vibration of the resonator. It is advantageous to measure very small amplitudes in the following circumstances:
1) Available driving force for the resonator is limited, either because of unavoidably large distance between the driving means and the resonator, or restricted available driving power (for instance, when operating in explosive atmospheres, to ensure intrinsically safe operation).
2) It is desired to operate a resonant fluid properties sensor over a very large range of amplitudes, so as to estimate non-Newtonian behavior of the fluid.
3) It is necessary to use soft magnetic materials rather than permanent magnets in the transducers driving and sensing the resonator. Such materials typically have much lower magnetic energy than do permanent magnets, requiring much more sensitive sensing means.
4) A fluid properties sensor with electromagnetic sensors must be operated in an environment where substantial magnetic fields are generated by nearby equipment such as electrical motors, transformers, and the like, such that said fields are capable of producing transducer outputs that would overwhelm the signals of interest.